The accelerator-pump system consists of three main components, the pump diaphragm, the pump cam, and the pump nozzle. This is the carburetor system that is most responsible for having good, crisp, off-idle throttle response. Its purpose is to inject a certain amount of fuel down the throttle bores when the throttle is opened to smooth the transition between the idle and main circuits so that no stumble, hesitation, or sluggishness will be evident during this transition phase.

The first adjustment to check is the clearance between the pump operating lever and the pump diaphragm cover’s arm at wide-open throttle. This ensures that the pump diaphragm is never stretched to its maximum limit at wide-open throttle, which will cause premature pump failure. The accelerator-pump arm is activated the moment that the throttle begins to move, which ensures instantaneous pump response. These adjustments can be made by simply turning the accelerator-pump adjusting screw located on the accelerator-pump arm together with the pump override spring and locknut.

The amount of fuel delivered by one accelerator-pump stroke is determined by the pump’s capacity and the profile of the pump cam. A larger pump delivery valve delivers the fuel much sooner than a smaller pump nozzle. During acceleration tests, if you notice that the car first hesitates and then picks up, it’s a sure bet the pump-nozzle size should be increased. A backfire (lean condition) on acceleration also calls for a larger pump-nozzle size. Conversely, if off-idle acceleration does not feel crisp or clean, the pump-nozzle size may need to be decreased. Holley accelerator-pump nozzles are stamped with a number that indicates the drilled pump hole size. Whenever a 0.040-inch or larger accelerator-pump nozzle is installed, the “hollow” pump-nozzle should also be used.

When changing the pump nozzle, it’s best to jump three sizes. Once nozzle size has been selected, the accelerator-pump system can be further tailored with the pump cam. Holley offers an assortment of different plunger pump cams, each with uniquely different lift and duration profiles that are available. The cam profile affects the movement of the accelerator-pump lever and, subsequently, the amount of fuel delivered by the pump nozzle.